Laser illumination is proving to be a promising illumination technology for projection displays because of its high colour gamut potential, low etendue and long lifetime. However, a major disadvantage of this technology is speckle in the image produced on the screen of the projection display due to the coherence of the laser. For example, it is well known from wave propagation theory that coherent wave sources (i.e. sources that are in phase) produce an interference pattern. A laser is a coherent source of light in which the wave components of the beam have a constant phase differential. When laser light is projected onto a screen for viewing, the waves reinforce and cancel each other thereby forming an interference pattern. To the viewer, this interference pattern appears as dark areas surrounded by many bright points of light in the projected image, an effect known as “speckle”. The actual points of light seem to be formed at the retina of the eye and not on the screen. Although the effect is perceived differently by different viewers, most viewers agree it is disturbing.
There are techniques that address speckle. These techniques generally involve methods of eliminating coherence through the use of oscillations of the laser, the screen or components of the projections system, or through the use of multi-fiber optical bundles. However, these techniques are complex, expensive and not entirely effective.
U.S. Pat. No. 6,625,381 discloses a speckle suppressed laser projection system wherein a semiconductor laser and an optical fiber are arranged such that a portion of the laser beam entering the optical fiber is reflected off the optical fiber and back into the semiconductor laser which induces the laser emission to change from a single mode to a multimode pattern. Each mode in the multimode pattern has a different speckle pattern, which blend together when projected on a screen to reduce perceived speckle. However, the invention is dependent on the precise arrangement of the laser and the fiber, and further the laser and the fiber must be maintained at a constant temperature. Hence, this approach is both complex and energy inefficient, and does not eliminate speckle, but merely reduces it.